This invention relates to a lubricating oil filter assembly of the type used on internal combustion engines. More specifically, the invention relates to a filter assembly having a spin-on or throwaway type filter cartridge adapted to be secured to a mounting adaptor on an engine having two lubricating oil circuits.
In such an engine, oil from the crankcase is filtered by a primary filter element and a substantial volume of the filtered oil flows to the primary lubricating circuit of the engine. A small percentage of the filtered oil is subjected to further filtration by a secondary filter element and is returned directly to the crankcase. The oil filtered by the secondary filter thus bypasses the main lubricating circuit and is returned to the crankcase in a cleaner condition than the oil flowing through the main circuit.
A filter cartridge for use with such an engine is disclosed in commonly assigned Cudaback U.S. application Ser. No. 411,700, filed Sep. 25, 1989. In such a cartridge, two filter elements are telescoped in coaxial relation with one another within an outer canister. Most of the oil passes through the outer filter to the primary lubricating circuit while some of the oil cleaned by the outer filter passes through the inner filter for further cleaning before flowing to the bypass circuit. A tubular fitting extends through end caps of the filter elements and connects with the mounting adaptor of the engine in order to discharge twice-filtered oil to the bypass circuit while keeping such oil separated from the oil flowing to the primary lubricating circuit. The mounting adaptor includes a stem-like bypass conduit which telescopes with the fitting to establish communication between the inner secondary filter and the bypass circuit.
When the engine is subjected to extremely dirty conditions, the service life of the filter cartridge may be unacceptably short even though the cartridge effects secondary filtration of the bypass oil. Under such circumstances, an external bypass circuit and filter unit may be provided to effect filtration of bypass oil only while a full flow filter cartridge attached to the mounting adaptor effects filtration only of the main lubricating oil. When an external bypass circuit is provided, however, it is necessary to lock out or disable the internal bypass circuit in the mounting adaptor in order to avoid possible starvation of the main lubricating circuit. Prior arrangements for locking out the internal bypass circuit are very expensive and require modification of the mounting adaptor of the engine.